A.c.power system couplings

ABSTRACT

In a short-circuit limiting coupling comprising a series combination of inductance and capacitance tuned to power-system frequency, at least a portion of the capacitance is connected in parallel with a bypass circuit which, at least in effect, comprises two parallel branches, one of the branches comprising a first saturable reactor in series with a first resistance shunted by a second saturable reactor, and the other branch comprising a second resistance in series with a rejector filter such as a parallel resonant circuit, also tuned to power-system frequency. The second reactor is designed to desaturate at a somewhat higher current level than the first; the first resistance will therefore assist in the recovery of the first reactor. The second resistance will damp harmonic and, more importantly, sub-harmonic oscillations.

[111 3,745,416 July 10, 1973 A. C. POWER SYSTEM COUPLINGS llemeshLaxmidas Thanawala, Stafford, England Assignee: Associated ElectricalIndustries Limited, London, England Filed: July 2, 1971 Appl. No.2159,375

Inventor:

[52] US. Cl 317/20, 317/50, 317/53,

323/77, 333/77, 333/79, 333/76 Int. Cl. H02h 9/00 Field of Search317/20, 53, 50, 16;

Electrical World; Short Circuit Limiter Offers Flexibility; Jan. 5,1970; pp. 50-51 Primary Examiner.l. D. Miller Assistant Examiner-HarveyFendelman Attorney- Keith Misegades and George R. Douglas Jr.

[57] ABSTRACT In a short-circuit limiting coupling comprising a seriescombination of inductance and capacitance tuned to power-systemfrequency, at least a portion of the capacitance is connected inparallel with a bypass circuit which, at least in efi'ect, comprises twoparallel branches, one of the branches comprising a first saturablereactor in series with a first resistance shunted by a second saturablereactor, and the other branch comprising a second resistance in serieswith a rejector filter such as a parallel resonant circuit, also tunedto power-system frequency. The second reactor is designed to desaturateat a somewhat higher current level than the first; the first resistancewill therefore assist in the recovery of the first reactor. The secondresistance will damp harmonic and, more importantly, subharmonicoscillations.

9 Claims, 12 Drawing Figures PATENIED JUL 101915 SHEEI 2 0F 2 A. C.POWER SYSTEM COUPLINGS This invention relates to electric circuit meansfor intercoupling two parts of an ac power system in such manner as tolimit the flow of current from one part to the other under abnormalsystem conditions, such as a short-circuit in the latter part. Suchmeans will be referred to herein as a short-circuit limiting coupling(or SLC).

The kind of SLC with which the invention is concerned comprises aseries-connected combination of inductance and capacitance having suchvalues that the combination is substantially series-resonant at thenormal power-system frequency (say, 50 or 60 Hz) and means foreffectively detuning the combination in response to over-currentconditions. Thus the coupling offers negligible or permissibly lowimpedance under normal system-operating conditions, but increases itsimpedance and limits the magnitude of current flow under abnormalconditions involving current flow of greater than normal magnitudethrough the coupling.

It is known (for example, from our British Pat. specification No.1,108,609) to provide an SLC of this kind in which the de-tuning meanscomprises a saturable re actor in parallel with at least a portion ofthe said capacitance. The saturable reactor is designed to beunsaturated and to allow very little current to flow through thisby-pass branch of the coupling under normal system-operating conditions,but to saturate and allow a substantial current to flow through thisbranch under over-current conditions. Under the latter conditions,therefore, the effective value of the capacitance and its associatedby-pass circuit is changed, and consequently the inductance-capacitancecombination is detuned so that its impedance at the power-systemfrequency is increased.

It is also known (for example, from our'British Pat. specifications Nos.1,108,608 and l,l46,676) to include resistance in such a by-pass branchto serve the dual purposes of damping out circuit transients andassisting the recovery of the saturable reactor to its unsaturated statewithin a short time after cessation of overcurrent conditions. Theresistance (or part of the resistance) may itself be by-passed by asaturable reactor or by an acceptor filter such as a series resonantcircuit tuned to the power-system frequency.

According to one aspect of the present invention, there is provided ashort-circuit limiting coupling including first capacitance means andfirst inductance means together forming a series resonant circuit at thefrequency of power supplied to the system, and circuit means connectedacross at least a portion of said capacitance means for detuning theresonant circuit in response to excess-current conditions, wherein saidcircuit means includes two parallel branches, a first branch comprising,in effect, first saturable reactance means, first resistance means fordamping said first reactance means and second saturable reactance meansfor controlling the effectiveness of said first resistance means, and asecond branch comprising, in effect, rejector filter means in serieswith second resistance means.

With the second saturable reactance means designed to desaturate atsomewhat higher current levels than the first saturable reactance means,the first resistance means will serve to assist the recovery of thefirst saturable reactance means. With the rejector filter tuned to saidfrequency, the second resistance means will serve to damp harmonic andsub-harmonic oscillations appearing across the circuit means. Thedamping of subharmonics is most important because rotating electricalmachines in the power system are otherwise likely to produce sustainedself-generated sub-harmonic oscillation.

The separation of the two damping functions (reactor damping andcapacitor damping) in this way offers the possibility of making the SLCcheaper and/or technically better for stability in particular. However,some of the components of the two effective branches can be combinedwith certain advantages, as will become apparent from the followingdescription of various embodiments of the invention, by way of example,with reference to the accompanying drawings (FIGS. 1 10).

Each of the Figures is a circuit diagram of a shortcircuit limitingcoupling comprising, between terminals 1 and 2, a main inductance 3 ofinductive reactance X1 and a main capacitance 4 of capacitive reactanceXc, constituting a series resonant circuit tuned to the normal frequency(say, 50 or 60 B2) of a power system in which the coupling is to be usedto interconnect two parts of the system: the capacitance 4 may of coursebe constituted by more than one capacitor. In parallel with a portion orpreferably, as shown, the whole of the capacitance 4 there is a by-passcircuit B which is different in the several embodiments.

In the SLC shown in FIG. 1 the by-pass circuit B consists of twoparallel branches B1 and B2. Branch Bl consists of a first saturablereactor 5 of reactance X2u when unsaturated, in series with a firstresistance 6 of ohmic value R3 shunted by a second saturable reactor 7of reactance X4u when unsaturated. Branch B2 consists of a secondresistance 8 of ohmic value Rd in series with a rejector filterconstituted by a parallel resonant circuit comprising an inductance 9 ofinductive reactance Xld and a capacitance 10 of capacitive reactanceXcd.

The values of Xld and Xcd are so chosen that the filter F is tuned tothe normal frequency of the power system. Branch B2 therefore has anegligible efi'ect on the operation of the coupling under normalconditions. However, the filter F offers low impedance to circuittransients and therefore brings the second resistor 8 into play to dampthe transients appearing across the main capacitor 4, including thosewhich arise from the other branch B1. The value of Rd is chosen toachieve such capacitor damping in an optimum manner.

The first saturable reactor 5 is designed so that its reactance X2uallows very little current to flow through branch Bl under normalsystem-operating conditions and so that it will saturate when thecurrent flowing through the coupling (and therefore the voltage,appearing across the main capacitance 4) exceeds a predetermined value.Thus this reactor functions as a magnetic switch. The second saturablereactor 7 is designed so that it will saturate at a slightly higherlevel of current in the coupling than the first but that, duringreduction of the current in branch B1 on recovery of the system, it willdesaturate at a somewhat higher current level than the first. Thus theresistance 6 will be brought into play to assist the recovery of thefirst saturable reactor 5. The value of R3 ischosen to achieve suchreactor damping in an optimum manner.

In the SLC shown in FIG. 2 the FIG. 1 arrangement is modified in thatthe reactor damping resistance 6 is replaced by a resistance 20 of ohmicvalue R311 which.

is suitable for both damping functions. The separate damping resistance8 is dispensed with and the rejector filter F is connected across thefirst saturable reactor 5. Since the optimum values of Rd and R3 in theFIG. 1 arrangement are different (Rd being usually larger than R3) thevalues of Xld and Xcd are smaller in the FIG. 2 arrangement to achievesimilar sub-harmonic performance. For the smaller value of Xcd thefilter capacitance has to be constituted by a larger number ofindividual capacitors in parallel, but this disadvantage is offset bythe saving in dispensing with the separate resistor 8. Moreover thewhole or a part of the filter F could now be placed, for convenience andeconomy, in the tank of an oil-immersed reactor constituting the firstsaturable reactor 5.

Although the two resistors have been combined into one in the FIG. 2arrangement, it will be understood that, functionally or effectively,the by-pass circuit B can still be regarded as comprising two parallelbranches one (as branch Bl) being the first reactor 5 in series with theresistance shunted by the second reactor 7, and the other (as branch B2)being the filter F in series with the resistance 20.

In the SLC shown in FIG. 3 the FIG. 1 arrangement is modified in thatthe second saturable reactor 7 is provided with a secondary winding 30and the reactordamping resistance 6 is connected across this winding.However this resistance is still, in effect, in series with the firstsaturable reactor 5 and shunted by the second 7. With this arrangementthe level of voltage across the resistance can be chosen to permit theuse of a resistor of lower ohmic value than would be required if it weredirectly connected: thus, for example, this arrangement may permit theuse of a steel resistor instead of a less reliable carbon resistor.Moreover, the use of low voltage winding with one end earthed (as shown)is particularly convenient in SLCs for higher system voltages, e.g. 132kV or 275 kV.

In the SLC shown in FIG. 4, the FIG. 1 arrangement is modified in boththe ways of the FIG. 2 and FIG. 3 arrangements i.e., a common dampingresistance 20 of ohmic value R3d is connected across a secondary winding30 of the second saturable reactor, and the filter F is connected acrossthe first saturable reactor 5. Alternatively, as indicated by the dottedlines 40 and 43, the filter F may be connected physically in series withthe resistance 20, the connections indicated by the full lines 41 and 44being omitted: then, as indicated by the dotted line 42, the resistance20, which acts directly as the capacitor damping resistance (Rd), isreflected to act as the reactor damping resistance (R3). Thisalternative permits the use of components having the same values of Xld,Xcd and Rd as in the FIG. 1 arrangement. Another alternative would haveconnections 40 and 44, and omit 41 and 43, the component values Xld andXcd then being different.

Further alternatives to the FIG. 4 arrangement are shown in FIG. 4A andFIG. 4B. In the FIG. 4A arrangement the common damping resistance 20 isconnected across the primary winding of the second reactor 7 while thesecondary winding 30 is connected in series with the filter F. In theFIG. 4B arrangement the second reactor 7 is provided with a thirdwinding 45 which is connected in series with the filter F.

In the SLC shown in FIG. 5 the FIG. 4 arrangement is modified in thatthe filter F is connected across a secondary winding 50 on the mainreactor 5. As the voltage provided by this winding can be suitablychosen and one end can be earthed (as shown) it is easier to design thefilter economically.

The SLC shown in FIG. 6 is similar to that of FIG. 5 except in that onlythe filter reactor 9 is connected across the secondary 50, while thefilter capacitance I0 is connected across the primary 5 of the mainreactor. The filter capacitance can therefore be formed of capacitors ofthe same design as those of the main bank constituting the maincapacitance 4.

The SLCs shown in the remaining Figures have a common feature whichdifferentiates them from those shown in the preceding Figures namely aparallel equivalent branch BIA in the by-pass circuit B. The secondsaturable reactor 7 shunts the first saturable reactor 5 as well as theresistance 6 in series with the latter. However, it has the same effectas if it were shunting the resistance 6 only. Of course it now becomeses sential to ensure that the second reactor 7 saturates at a highercurrent level of the coupling than the first 5, as well as desaturatingat a higher current level than the first.

In the SLC's shown in FIGS. 7 and 8 the second reactor 7 has a singlewinding. In those shown in FIGS. 9 and 10 the second reactor 7 has asecondary winding across which the series combination of first reactor 5and resistance 6 is connected; in other words, the second reactor isconstituted by a saturating transformer with thesame knee-point voltageas the reactor 7 of FIGS. 7 and 8. These arrangements are functionallyor effectively equivalent but the provision of the secondary windingenables the voltage across the latter to be chosen to make it easier todesign ecoomically the components connected thereto: with one end of thesecondary winding 90 earthed, the FIG. 10 arrangement is a particularadvantageous form of SLC for high voltage systems (e.g. 132 kV or 275 kVor higher voltages).

Each of the SLCs of FIGS. 7 10 includes, at least in effect, a capacitordamping branch as in the SLCs of FIGS. 1 6, consisting of a rejectorfilter F in series with a resistance. In FIGS. 7 and 9 this resistance 8(of ohmic value Rd) is separate from the reactor damping resistance 6(of ohmic value R3). In FIGS. Sand 10 there is a common resistance 20(of ohmic value R311) as in FIG. 2, and the values of Xld and Xcd aredifferent from those of FIG. 7 as is the case between FIG. 2 and FIG. 1.

It will be understood that there are various other possibilities forderiving different combinations from those which have been described, onthe basis of the principles which have been mentioned.

I claim:

1. An a.c. power system coupling including first capacitance means andfirst inductance means together forming a series resonant circuit at thefrequency of the power supplied to the system, circuit means fordetuning the resonant circuit in response -to excesscurrent conditionsand comprising a first saturable reactance means operatively associatedwith a current path shunting at least part of the first capacitancemeans so as to be unsaturated and to allow very little current to flowtherethrough under normal operating conditions, but to allow asubstantial current to flow therethrough under excess-currentconditions, rejector filter means connected so as to shunt at least partof the a first capacitance means and tuned to the frequency of the powersupplied to the system, a second saturable reactance means in the formof a saturable transformer having a primary winding connected in serieswith the first saturable reactance means and the rejector filter means,and resistance means for damping the first saturable reactance meansconnected across a secondary winding of the saturable transformer so asalso to provide a resistance effectively in series with the rejectorfilter means to damp harmonic and sub-harmonic oscillations.

2. An a.c. power system coupling including first capacitance means andfirst inductance means together forming a series resonant circuit at thefrequency of the power supplied to the system, circuit means fordetuning the resonant circuit in response to excesscurrent conditionsand comprising a first saturable reactance means operatively associatedwith a current path shunting at least part of the first capacitancemeans so as to be unsaturated and to allow very little curent to flowtherethrough under normal operating conditions, but to allow asubstantial current to flow therethrough under excess-currentconditions, rejector filter means connected so as to shunt at least partof the first capacitance means and tuned to the frequency of the powersupplied to the system, a second saturable reactance means in the formof a saturable transformer having a primary winding connected in serieswith the first saturable reactance means, and a secondary windingconnected in series with the rejector filter means across at least partof the first saturable reactance means, and resistance means for dampingthe first saturable reactance means connected across the primary windingof the saturable transformer so as also to provide a resistanceeffectively in series with the rejector filter means to damp harmonicand sub-harmonic oscillations.

3. An a.c. power system coupling including first capacitance means andfirst inductance means together forming a series resonant circuit at thefrequency of the power supplied to the system, circuit means fordetuning the resonant circuit in response to excesscurrent conditionsand comprising a first saturable reactance means operatively associatedwith a current path shunting at least part of the first capacitancemeans so as to be unsaturated and to allow very little current to flowtherethrough under normal operating conditions, but to allow asubstantial current to flow therethrough under excess-currentconditions, rejector filter means connected so as to shunt at least partof the first capacitance means and tuned to the frequency of the powersupplied to the system, a second saturable reactance means in the formof a saturable transformer having a primary winding connected in serieswith the first saturable reactance means, a first secondary windingconnected in series with the rejector filter means across at least partof the first capacitance means, and resistance means for damping thefirst saturable reactance means connected across a further secondarywinding of the transformer so as also to provide a resistanceeffectively in series with the rejector filter means to damp harmonicand sub-harmonic oscillations.

4. An a.c. power system coupling including first capacitance means andfirst inductance means together forming a series resonant circuit at thefrequency of the power supplied to the system, circuit means fordetuning the resonant circuit in response to excesscurrent conditionsand comprising a first saturable reactance means in the form of asaturable transformer having its primary winding shunting the firstcapacitance means and arranged to be unsaturated to allow very littlecurrent to flow therethrough under normal operating conditions, but toallow a substantial current to flow therethrough under excess-currentconditions, a rejector filter means at least part of which is connectedacross a secondary winding of the transformer so that it effectivelyshunts the first capacitance means and which is tuned to the frequencyof the power supplied to the system, a second saturable reactance meansin the form of a further saturable transformer having its primarywinding connected in series with the primary winding of the firstsaturable reactance means across the first capacitance means, andresistance means for damping the first saturable reactance meansconnected across a secondary winding of the further saturabletransformer so as also to provide a resistance effectively in serieswith the rejector filter means to damp harmonic and sub-harmonicoscillations.

5. A system as claimed in claim 4 wherein a capacitance forming part ofthe rejector filter means is connected across the primary winding of thesaturable transformer forming the first saturable reactor means.

6. An a.c. power system coupling including first capacitance means andfirst inductance means together forming a series resonant circuit at thefrequency of the power supplied to the system, circuit means fordetuning the resonant circuit in response to excesscurrent conditionsand comprising a first saturable reactance means operatively associatedwith a current path shunting at least part of the first capacitancemeans so as to be unsaturated and to allow very little current to flowtherethrough under normal operating conditions, but to allow asubstantial current to flow therethrough under excess-currentconditions, rejector filter means connected so as to shunt at least partof the first capacitance means and tuned to the frequency of the powersupplied to the system, a second saturable reactance means in parallelwith the first saturable reactance means and with the rejector filtermeans, and resistance means comprising a first resistance connected inseries with the first saturable reactance means for damping the latterand a second resistance connected in series with the rejector filtermeans for damping harmonic and sub-harmonic oscillations.

7. An a.c. power system coupling including first capacitance means andfirst inductance means together forming a series resonant circuit at thefrequency of the power supplied to the system, circuit means fordetuning the resonant circuit in response to excesscurrent conditionsand comprising a first saturable reactance means operatively associatedwith a current path shunting at least part of the first capacitancemeans so as to be unsaturated and to allow very little current to flowtherethrough under normal operating conditions, but to allow asubstantial current to flow therethrough under excess-currentconditions, rejector filter means tuned to the frequency of the powersupplied to the system and connected across the first saturable reactormeans, a second saturable reactor means connected in parallel with thefirst saturable reactor means, and resistance means in series with thefirst saturable reactor means and the rejector filter for dampingharmonic and sub-harmonic oscillations.

8. An ac power system coupling including first'capacitance means andfirst inductance means together forming a series resonant circuit at thefrequency of the power supplied to the system, circuit means fordetuning the resonant circuit in response to excesscurrent conditionsand comprising a first saturable reactance means. operatively associatedwith a current path shunting at least part of the first capacitancemeans so as to be unsaturated and to allow very little current to flowtherethrough under normal operating conditions, but to allow asubstantial current to flow therethrough under excess-currentconditions, rejector filter means connected so as to shunt at least partof the first capacitance means and tuned to the frequency of the powersupplied to the system, a second saturable reactance means in the formof a saturable transformer having a primary winding connected across atleast part of the first capacitance, the first saturable reactance meansand the rejector filter means being connected in parallel across asecondary winding of the transformer, and resistance means comprising afirst resistance connected in series with the first saturable reactancemeans for damping the latter and a second resistance connected in serieswith the rejector filter means for damping harmonic and sub-harmonicoslillations.

9. An ac power system coupling including first capacitance means andfirst inductance means together forming a series resonant circuit at thefrequency of the power supplied to the system, circuit means fordetuning the resonant circuit in response to excess current conditionsand comprising a first saturable reactance means operatively associatedwith a current path shunting at least part of the first capacitancemeans so as to be unsaturated and to allow very little current to flowtherethrough under normal operating conditions, but to allow asubstantial current to flow therethrough under excess-currentconditions, rejector filter means connected so as to shunt at least partof the first capacitance means and tuned to the frequency of the powersupplied to the system, a second saturable reactance means in the formof a saturable transformer having a primary winding connected across atleast part of the first capacitance, the first saturable reactance monicand sub-harmonic oscillations.

1. An a.c. power system coupling including first capacitance means andfirst inductance means together forming a series resonant circuit at thefrequency of the power supplied to the system, circuit means forde-tuning the resonant circuit in response to excess-current conditionsand comprising a first saturable reactance means operatively associatedwith a current path shunting at least part of the first capacitancemeans so as to be unsaturated and to allow very little current to flowtherethrough under normal operating conditions, but to allow asubstantial current to flow therethrough under excess-currentconditions, rejector filter means connected so as to shunt at least partof the first capacitance means and tuned to the frequency of the powersupplied to the system, a second saturable reactance means in the formof a saturable transformer having a primary winding connEcted in serieswith the first saturable reactance means and the rejector filter means,and resistance means for damping the first saturable reactance meansconnected across a secondary winding of the saturable transformer so asalso to provide a resistance effectively in series with the rejectorfilter means to damp harmonic and sub-harmonic oscillations.
 2. An a.c.power system coupling including first capacitance means and firstinductance means together forming a series resonant circuit at thefrequency of the power supplied to the system, circuit means forde-tuning the resonant circuit in response to excess-current conditionsand comprising a first saturable reactance means operatively associatedwith a current path shunting at least part of the first capacitancemeans so as to be unsaturated and to allow very little curent to flowtherethrough under normal operating conditions, but to allow asubstantial current to flow therethrough under excess-currentconditions, rejector filter means connected so as to shunt at least partof the first capacitance means and tuned to the frequency of the powersupplied to the system, a second saturable reactance means in the formof a saturable transformer having a primary winding connected in serieswith the first saturable reactance means, and a secondary windingconnected in series with the rejector filter means across at least partof the first saturable reactance means, and resistance means for dampingthe first saturable reactance means connected across the primary windingof the saturable transformer so as also to provide a resistanceeffectively in series with the rejector filter means to damp harmonicand sub-harmonic oscillations.
 3. An a.c. power system couplingincluding first capacitance means and first inductance means togetherforming a series resonant circuit at the frequency of the power suppliedto the system, circuit means for de-tuning the resonant circuit inresponse to excess-current conditions and comprising a first saturablereactance means operatively associated with a current path shunting atleast part of the first capacitance means so as to be unsaturated and toallow very little current to flow therethrough under normal operatingconditions, but to allow a substantial current to flow therethroughunder excess-current conditions, rejector filter means connected so asto shunt at least part of the first capacitance means and tuned to thefrequency of the power supplied to the system, a second saturablereactance means in the form of a saturable transformer having a primarywinding connected in series with the first saturable reactance means, afirst secondary winding connected in series with the rejector filtermeans across at least part of the first capacitance means, andresistance means for damping the first saturable reactance meansconnected across a further secondary winding of the transformer so asalso to provide a resistance effectively in series with the rejectorfilter means to damp harmonic and sub-harmonic oscillations.
 4. An a.c.power system coupling including first capacitance means and firstinductance means together forming a series resonant circuit at thefrequency of the power supplied to the system, circuit means forde-tuning the resonant circuit in response to excess-current conditionsand comprising a first saturable reactance means in the form of asaturable transformer having its primary winding shunting the firstcapacitance means and arranged to be unsaturated to allow very littlecurrent to flow therethrough under normal operating conditions, but toallow a substantial current to flow therethrough under excess-currentconditions, a rejector filter means at least part of which is connectedacross a secondary winding of the transformer so that it effectivelyshunts the first capacitance means and which is tuned to the frequencyof the power supplied to the system, a second saturable reactance meansin the form of a further saturable transformer having its primarywinding connectEd in series with the primary winding of the firstsaturable reactance means across the first capacitance means, andresistance means for damping the first saturable reactance meansconnected across a secondary winding of the further saturabletransformer so as also to provide a resistance effectively in serieswith the rejector filter means to damp harmonic and sub-harmonicoscillations.
 5. A system as claimed in claim 4 wherein a capacitanceforming part of the rejector filter means is connected across theprimary winding of the saturable transformer forming the first saturablereactor means.
 6. An a.c. power system coupling including firstcapacitance means and first inductance means together forming a seriesresonant circuit at the frequency of the power supplied to the system,circuit means for de-tuning the resonant circuit in response toexcess-current conditions and comprising a first saturable reactancemeans operatively associated with a current path shunting at least partof the first capacitance means so as to be unsaturated and to allow verylittle current to flow therethrough under normal operating conditions,but to allow a substantial current to flow therethrough underexcess-current conditions, rejector filter means connected so as toshunt at least part of the first capacitance means and tuned to thefrequency of the power supplied to the system, a second saturablereactance means in parallel with the first saturable reactance means andwith the rejector filter means, and resistance means comprising a firstresistance connected in series with the first saturable reactance meansfor damping the latter and a second resistance connected in series withthe rejector filter means for damping harmonic and sub-harmonicoscillations.
 7. An a.c. power system coupling including firstcapacitance means and first inductance means together forming a seriesresonant circuit at the frequency of the power supplied to the system,circuit means for de-tuning the resonant circuit in response toexcess-current conditions and comprising a first saturable reactancemeans operatively associated with a current path shunting at least partof the first capacitance means so as to be unsaturated and to allow verylittle current to flow therethrough under normal operating conditions,but to allow a substantial current to flow therethrough underexcess-current conditions, rejector filter means tuned to the frequencyof the power supplied to the system and connected across the firstsaturable reactor means, a second saturable reactor means connected inparallel with the first saturable reactor means, and resistance means inseries with the first saturable reactor means and the rejector filterfor damping harmonic and sub-harmonic oscillations.
 8. An a.c. powersystem coupling including first capacitance means and first inductancemeans together forming a series resonant circuit at the frequency of thepower supplied to the system, circuit means for de-tuning the resonantcircuit in response to excess-current conditions and comprising a firstsaturable reactance means operatively associated with a current pathshunting at least part of the first capacitance means so as to beunsaturated and to allow very little current to flow therethrough undernormal operating conditions, but to allow a substantial current to flowtherethrough under excess-current conditions, rejector filter meansconnected so as to shunt at least part of the first capacitance meansand tuned to the frequency of the power supplied to the system, a secondsaturable reactance means in the form of a saturable transformer havinga primary winding connected across at least part of the firstcapacitance, the first saturable reactance means and the rejector filtermeans being connected in parallel across a secondary winding of thetransformer, and resistance means comprising a first resistanceconnected in series with the first saturable reactance means for dampingthe latter and a second resistance connected in series witH the rejectorfilter means for damping harmonic and sub-harmonic oscillations.
 9. Ana.c. power system coupling including first capacitance means and firstinductance means together forming a series resonant circuit at thefrequency of the power supplied to the system, circuit means forde-tuning the resonant circuit in response to excess-current conditionsand comprising a first saturable reactance means operatively associatedwith a current path shunting at least part of the first capacitancemeans so as to be unsaturated and to allow very little current to flowtherethrough under normal operating conditions, but to allow asubstantial current to flow therethrough under excess-currentconditions, rejector filter means connected so as to shunt at least partof the first capacitance means and tuned to the frequency of the powersupplied to the system, a second saturable reactance means in the formof a saturable transformer having a primary winding connected across atleast part of the first capacitance, the first saturable reactance meansand the rejector filter means being connected in parallel across asecondary winding of the transformer, and resistance means in serieswith the first saturable reactor means and the rejector filter fordamping harmonic and sub-harmonic oscillations.